NUMERICAL MODELLING OF PEAT BEHAVIOUR Farrell … · NUMERICAL MODELLING OF PEAT BEHAVIOUR ... PRESENTATION ¾Review some ... Soft Soil Creep 10.5 0.2 0.04 0.009 12.15 Osario‐Salas

NUMERICAL MODELLING OF PEAT BEHAVIOUR

Eric R Farrell

AGL Consulting and TrinityAGL Consulting and Trinity College, Dublin

ObjectiveObjective

• Look at what the theoretical/laboratory studies oo at at t e t eo et ca / abo ato y stud escarried out to date add to practical design situations.

• This is addressed by taking an example of a floating road to take a heavy mobile load

• It has obvious relevance to the roads used in windfarm developments.

11/09/2013 ET Hanrahan Memorial Symposium 2

PRESENTATION

Review some special aspects of pseudo fibrous peat b h ibehaviour

Outline some experience of floating roads on peatOutline some experience of floating roads on peat

Compare practical experience with numerical model p p ppredictions.

This comparison will be done with existing soil models, acknowledging that specially adapted soil models are being developed in research.g p


ET Hanrahanh d f hAhead of his time in peat

• Lightweight fill (bales of g g (peat)

• Vacuum consolidation trials

• Effectiveness of surchargesurcharge

• Many other areas.Many other areas.


SOME SPECIAL ASPECTS OF PSEUDO FIBROUS PEAT

Very low ρ – effective stress close to zero in many design situationssituations

Drained compression tests are essentially 1‐D (very low μ′) (Hebib & Farrell 1998 O’Kelly & Zhang 2013 )(Hebib & Farrell 1998, O Kelly & Zhang, 2013 )

Triaxial compression at low effective stresses, σ3f'≈ 0 ( ?? b b )(Termaat, ??, Hebib, 1997)

High φ’ in vertical compression, φ’ depends on direction of g φ p , φ ploading.

Anisotropic strength and stiffnessAnisotropic strength and stiffness


Special aspect (cont)

High compressibilityHigh compressibility

Time dependent deformations (Den Haan 1992)Time dependent deformations (Den Haan, 1992)

Effect of structure (Zhang and O’Kelly, 2013)

Permeability dependent on void ratio (Hanrahan, 1954)


SOME PRACTICAL EXPERIENCEOF FLOATING ROADS ON PEAT


Floating road test Index test ‐ 65mmx 130mm vane versus 130mm x 220mm

Undrained Shear Strength c (kN/m2)

VST Results Max. height 2m in 8 No. liftsPeat depth 4 to 5m ave 4 65m

0.00

0.50

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20Undrained Shear Strength, cu (kN/m ) Peat depth 4 to 5m, ave. 4.65m

Tree trunks 5m long and dia. <0.5mTensar geogrid and 500mm angular stone

1.00

1.50

2.00

400

500

600

2.50

3.00

3 50

Dep

th (m

)

200

300

400Settlemen

t (mm)

3.50

4.00

4.500

100

0 0.5 1 1.5 2 2.5

S


5.00 Geonor-H10 West Geonor-H10 East Geonor H10 Average ESBi Vane WestESBi Vane East ESBi Vane Average

Height of crushed rock (m)

Typical profile through raised bog.Osorio‐Salas ‐ 2012

w (%) LOI

0.0

0.5

0% 500% 1000% 1500%

( )

0.0

0.5

0% 50% 100%

1.0

1.5

2.0

(m)

1.0

1.5

2.0

(m)

Transition zone –2.5

3.0

3.5

4 0

Dep

th

2.5

3.0

3.5

4 0Dep

th

important in some design situations

4.0

4.5

5.0

BH 1 BH 2 MBH 3 MBH 4

4.0

4.5

5.0

BH 1 BH 2 MBH 3 MBH 4


BH‐1 BH‐2 MBH‐3 MBH‐4 BH‐1 BH‐2 MBH‐3 MBH‐4

NOTE:‐ CASE STUDY IS ON BLANKET BOG BUT WITH SIMILAR MOISTURE CONTENTS

0.7

0.8

0.9m

)

0 3

0.4

0.5

0.6

abov

e bt

m o

f pea

t.(m

0

0.1

0.2

0.3

0 200 400 600 800 1000 1200

Hei

ght

Moisture content %

Edge of slip Edge of slip after test


Settlement parameters used in the peat models generallySettlement parameters used in the peat models generally obtained from work of Osorio‐Salas (2012) on raised bog which found good comparison with field performance ith l b t bt i d twith laboratory obtained parameters.


Selection of effective stress fparameters of Peat

• c' ≈ 0 kPa• φ‘ = 50o and 35o (anisotropic)

Actually required c' ≈ 3 kPa to 5kPa

Due to effect of fibres but perhaps it matches the s profile from vane

ET Hanrahan Memorial Symposium 12

su profile from vane.

Selection of stiffness parametersCase Histories

E=200kPa

E=75kPa


Landva & Rochelle, 1983

Stiffness parameters for soil modelsStiffness parameters for soil modelsModel γ

kN/m3E'

kPaμ' Source

Elastic M‐C 10.5 350 0.1 To give approx. same strain at 200kPa as lab. Test (ignores stress dependency of E')

Model γkN/m3

λ* κ* μ* eo Source

Soft Soil 10.5 0.19 0.04 N/A 12.15 Osario‐Salas (2010)

Soft Soil Creep

10.5 0.2 0.04 0.009 12.15 Osario‐Salas (2010)Creep


Stiffness parameters cont.Stiffness parameters cont.

Model γ m Eref50 Erefoed Erefur ur SourcekN/m3 kPa kPa kPa

H‐S 10.5 1.0 530 530 4500 0.2 See below

Hand calculations :‐ eo=12.15; Cc = 6.1 ; Cs=0.8, σc‘ = 9kPa


500

600

300

400

E‘ kPa

0

100

200

O'Kelly & Zhang, 2013

O’Kelly & Zhang, 2013 0

0 20 40 60 80 100 120

p' kPa

Eref using 500kPa O'K&Z

Ref

O Kelly & Zhang, 2013


Apparent preconsolidation pressure of peat

• Uncertainty regarding the apparent preconsolidation pressure

• 9kPa & 9.5kPa from lab. Tests (Osario‐Salas‐2010)

• Published information ≈ 8 to 10 kPa (Farrell, 2012)

UPDATED MESH OPTION???


PermeabilityPermeability

BALLYDERMOT BOG - BOREHOLE 1 ko= 0.0023m/day

9101112131415

ko 0.0023m/day2.6x10‐8 m/sCk =3.44

3456789

e

D=0.90-1.55m

kave= 10‐8 m/s

012

1.00E-11 1.00E-10 1.00E-09 1.00E-08 1.00E-07 1.00E-06 1.00E-05

kv log (m/s)

D=1.50-3.20m

Log. (D=0.90-1.55m)

Log. (D=1.50-3.20m)

v-log ( )


Comparison with lab. resultsComparison with lab. results

Stress Stress

0.00

10.00

0 50 100 150 200 250 300

0.00

10.00

1 10 100

20.00

30.00

train %

20.00

30.00

train %

40.00

50.00

60.00

S 40.00

50.00

60.00S

70.00

2.4m to 3.2m Oed 4 Spreadsheet

Soft Soil SSCreep

70.00

2.4m to 3.2m Oed 4 Spreadsheet

Soft Soil SSCreep


HS Lin E M‐C HS Lin E M‐C

UNDRAINED TRIAXIAL q/p' plotsUNDRAINED TRIAXIAL q/p plots60.00

40.00

50.00

M C 10kPa

30.00

q kPa

M‐C 10kPa

M‐C 40kPa

SS 10kPa

SS40

SSC 10kPa

SSC 40kP

10.00

20.00 SSC 40kPa

HS 10Kpa

HS 40kPa

0.00

0.00 10.00 20.00 30.00 40.00 50.00 60.00

p' kPa


s /σ3 'su/σ3c

35

25

30

15

20

s ukPa

M‐C

Soft Soil

SSC

5

10 HS

0

0 5 10 15 20 25 30 35 40 45

σ3c'


Undrained stress vs strainlTriaxial compression

70

50

60

30

40

σ 1‐σ

3

M‐C

HS

20

30SS

SSC

0

10

0 5 10 15 20 25 30 35


0 5 10 15 20 25 30 35

Axial strain %

Modelling Floating RoadModelling Floating Road


Parameters used to model floating droad

Model Length(m)

E GPa

Trunkdia.

EAkN/m

EIkNm2/m

WkN/m/m

Timber Plate 5 13 0.3m 3063060 17230 6.2

Model E' μ' c' φ' γkPa kN/m2

Gravel ElasticM‐C

60000 0.2 1 45 20


Construction process modelled.Construction process modelled.

• Allowed to consolidate after construction ofAllowed to consolidate after construction of road.

• Two loading scenarios considered:‐– Loading completely undrained

– Loading applied over 0.5 days with consolidation.


FEA Modelling of load test – 30kPad d dLoading stage undrained


Observations at centrelineObservations at centrelineVertical settlement are uniform

4 5

5

0 10 20 30

σyy'kPa

0 10 20 30

σyy'kPa

Effective stresses are low, as expected

Deformations sensitive to pre‐3.5

4

4.50

0.5

1

0

0.5

1Deformations sensitive to preconsolidation pressure.

c' = 3 to 5 kPa required for stability h φ‘

2.5

3

Dep

th (m

)

Cons 0.5day

No cons

1.5

2

m)

Floating Rd only

30kPa No cons

1

1.5

2

)

Floating Rd only

30kPa No cons

30kPa 0.5d conswith φ‘ = 35o

φ‘ = 50o lesser benefit.1

1.5

2No cons

Floating Rd2.5

3

3 5

Dep

th (m 30kPa 0.5d cons

2.5

3Dep

th (m

)

After 30d

Full cons

0

0.5

1

0 10 20 30

3.5

4

4.5

3.5

4

4.5


0 10 20 30

σyy'55

30

350

15

20

25

30

ertical Stress (kPa

0

No cons

Soft Soil Model

0

5

10

0 1 2 3 4 5

Total V

Distance along Floating Road (m)

200

250

)

100

150

200

Displacem

ent (mm

No cons

0

50Vertical D No cons


0 1 2 3 4 5

Distance along Floating Road (m)

500

600

400

mm) SS ‐ No cons

200

300

Settlemen

t (m SS‐wt cons

SSC ‐No cons

SSC wt Cons

Measured

M C No cons

100

200 M‐C No cons

M‐C Cons

0

0 5 10 15 20 25 30 35

Applied Stress on road surface kPaSettlement after 24hrs was about


24hrs was about 0.9m

CommentsCommentsNEED TO IDENTIFY SOURCES OF THE DIFFERENCE BETWEEN FIELD PERFORMANCE AND NUMERICAL PREDICTIONS

600

400

500

nt (m

m)

SS ‐ No cons

SS‐wt cons

SSC ‐No cons Soft Soil model ith ‘ 7kP

100

200

300

Settlemen SSC wt Cons

Measured

M‐C No cons

M‐C Cons

with σc‘ = 7kPa(with cons)

0

100

0 5 10 15 20 25 30 35

Applied Stress on road surface kPa

M C Cons

SS‐ 7kPa‐ wt consLarge plastic strains with σc‘ = 6kPa.


Time/Settlement under 30kPaTime/Settlement under 30kPa

Time (days)

0.00

100.00

1 10 100 1,000

Time (days)

200.00

300.00

mm

)

SS Full cons

400.00

500.00

tlem

ent (

m SSC

m‐c

600.00

700.00

Sett


800.00

900.00

Some points made by O’Loughlin(2001) based on lab. and field t distudies.

Summary of testSummary of test

i ld l d li d 9k 2 b• Field load test applied 7.9kPa over a 5.2m by 4.175m area using large capacity water tanks ( l h h b f h k i h i(although submergence of the tanks with time reduced the effective stress). The tanks settled b 860 iabout 860mm in two years.


O’L hli 2001


O’Loughlin, 2001

CONCLUSIONSCONCLUSIONS• Virgin pseudo fibrous peat has many unique features which make it difficult to modelwhich make it difficult to model.

• Numerical modelling can assist in interpreting the behaviour of peat but further advances are required.

• More long term field monitoring is required to feed• More long term field monitoring is required to feed into the numerical modelling developments.

• Given the prevalence of floating roads for windfarms, data on their performance, preferably with comparison with predictions, should be collected for p presearch.


ReferencesReferences

• Farrell & Hebib, 1998, The determination of the geotechnical ti f i il P bl ti il fproperties of organic soils, Problematic soils conf.

• Zang and O’Kelly, 2013, The principle of effective stress and triaxialcompression testing of peatcompression testing of peat.

• O’Kelly and Zang, 2013, Consolidated‐drained triaxial compression testing of peat Geotechnical Testing Jnltesting of peat, Geotechnical Testing Jnl.

• Hanrahan, 1954, An investigation into some physical properties of peat Geotechniquepeat. Geotechnique

• Den Haan, 1992, The formulation of Virgin Compression of Soils, Geotechnique., q


References contd.

• Hebib, 1997, Study of the shear strength of peat under different stress paths MSc thesis TCDpaths, MSc thesis, TCD.

• Osorio‐Salas, 2012, Vacuum consolidation field test on a pseudo‐fibrous peat, PhD Thesis, TCD.p , ,

• Hebib, 2001, Experimental investigation on the stabilisation of Irish peat. PhD Thesis, TCD.

• O’Loughlin, 2001, The one‐dimensional compression of fibrous peat and other organic soils., PhD Thesis, TCD.

• O’Loughlin & Lehane, 2001, Modelling one dimensional compression of a fibrous peat, 15th ISSMGE

•• .


Documents

NUMERICAL MODELLING OF PEAT BEHAVIOUR Farrell … · NUMERICAL MODELLING OF PEAT BEHAVIOUR ... PRESENTATION ¾Review some ... Soft Soil Creep 10.5 0.2 0.04 0.009 12.15 Osario‐Salas